This invention relates generally to an apparatus and method for determining an IR spectrum of a sample material. More particularly, the disclosed invention relates to spectroscopically determining the IR spectrum of a sample using an apparatus and method that operate in real-time, and which do not require the use of any moving parts. Still further, the apparatus and method of the disclosed invention do not require extensive mathematical transformation of the detected spectral information to analyze the composition of the sample material.
The disclosed invention has industrial applicability to, for example, a real-time method to monitor manufacturing processes. Such processes include, but are not limited to measurement of thickness, chemical structure, and orientation of coatings on surfaces (solid, liquid, chemically bound, physically adsorbed). These measurements include, but are not limited to those made on biological materials, polymers, superconductors, semiconductors, metals, dielectrics, and minerals. Further applicability is found to a real-time apparatus and method to measure and detect a chemical species present in a chemical reaction involving various processing of materials in any of a gaseous, liquid, or solid state.
As industry continues on its path of cost reductions in core technologies, more emphasis will be placed on the optimization of processes and performance. This retrenchment will necessitate the development and introduction of a whole new class of sophisticated instrumentation that is portable, rugged, reliable, and capable of operation over long periods of time in an aggressive industrial or other non-laboratory environment.
Spectrometric techniques are often used in analysis of materials. Classically, spectroscopy is the measurement of the selective absorption, emission, or scattering of light (energy) of specific colors by matter. Visible white light can be separated into its component colors, or spectrum, by a prism, for example. The principal purpose of a spectroscopic measurement is usually to identify the chemical composition of an unknown material, or to elucidate details of the structure, motion, or environmental characteristics (e.g., internal temperature, pressure, magnetic field strength, etc.) of a xe2x80x9cknownxe2x80x9d material or object. Spectroscopy""s widespread technical importance to many areas of science and industry can be traced back to nineteenth-century successes, such as characterizing natural and synthetic dyes, and determining the elemental compositions of stars.
Modern applications of spectroscopy have generalized the meaning of xe2x80x9clightxe2x80x9d to include the entire range or spectrum of electromagnetic radiation, which extends from gamma- and x-rays, through ultraviolet, visible, and infrared light, to microwaves and radio waves. All these various forms (or wavelength ranges) of electromagnetic radiation have their own characteristic methods of measurement. These different methods give rise to various types of spectroscopic apparatus and techniques that are outwardly very different from each other, and which often rely upon difference physical phenomena to make measurements of material characteristics. Further, the various experts and other researchers in these diverse fields, more often than not, do not cross the technical boundaries between these areas of specialization, as different and somewhat compartmentalized knowledge bases and xe2x80x9crules of thumbxe2x80x9d are used.
The use of infrared (IR) is one of numerous spectroscopic techniques for analyzing the chemistry of materials. In all cases, spectroscopic analysis implies a measurement of a very specific wavelength of light energy, either in terms of the amount absorbed or reflected by the sample in question, or the amount emitted from the sample when suitably energized.
In the case of IR, an absorption form of spectrometric analysis is relied upon. IR radiation does not have enough energy to induce transitions between different electronic states, i.e., between molecular orbitals, as seen with ultraviolet (UV), for example. Unlike atomic absorption, IR spectroscopy examines vibrational transitions within a single electronic state of a molecule, and is not concerned with specific elements, such as Pb, Cu, etc. Such vibrations fall into one of three main categories, i.e., stretching, which results from a change in inter-atomic distance along the bond axis; bending, which results from a change in the angle between two bonds; and torsional coupling, which relates to a change in angle and separation distance between two groups of atoms. Almost all materials absorb IR radiation, except homonuclear diatomic molecules, e.g., O2, H2, N2, Cl2, F2, or noble gases.
IR typically covers the range of the electromagnetic spectrum between 0.78 and 1000 xcexcm. Within the context of IR spectroscopy, temporal frequencies are measured in xe2x80x9cwavenumbersxe2x80x9d (in units of cmxe2x88x921), which are calculated by taking the reciprocal of the wavelength (in centimeters) of the radiation. Although not precisely defined, the IR range is sometimes further delineated by three regions having the wavelength and corresponding wavenumber ranges indicated:
For a molecule to absorb IR, the vibrations or rotations within the molecule must cause a net change in the dipole moment of the molecule. The alternating electric field of the incident IR radiation interacts with fluctuations in the dipole moment of the molecule and, if the frequency of the radiation matches the vibrational frequency of the molecule, then radiation will be absorbed, causing a reduction in the IR band intensity due to the molecular vibration.
An electronic state of a molecular functional group may have many associated vibrational states, each at a different energy level. Consequently, IR spectroscopy is concerned with the groupings of atoms in specific chemical combinations to form what are known as xe2x80x9cfunctional groupsxe2x80x9d, or molecular species. These various functional groups help to determine a material""s properties or expected behavior by the absorption characteristics of associated types of chemical bonds. These chemical bonds undergo a change in dipole moment during a vibration. Examples of such functional groups and their respective energy bands include, for example, hydroxl (Oxe2x80x94H) (3610-3640 cmxe2x88x921), amines (Nxe2x80x94H) (3300-3500 cmxe2x88x921), aromatic rings (Cxe2x80x94H) (3000-3100 cmxe2x88x921), alkenes (Cxe2x80x94H) (3020-3080 cmxe2x88x921), alkanes (Cxe2x80x94H) (2850-2960 cmxe2x88x921), nitrites (C=-N) (2210-2260 cmxe2x88x921), carbonyl (Cxe2x95x90O) (1650-1750 cmxe2x88x921), or amines (Cxe2x80x94N) (1180-1360 cmxe2x88x921). The IR absorption bands associated with each of these functional groups act as a type of xe2x80x9cfingerprintxe2x80x9d which is very useful in composition analysis, particularly for identification of organic and organometallic molecules.
By knowing which wavelengths are absorbed by each functional group of interest, an appropriate wavelength can be directed at the sample being analyzed, and then the amount of energy absorbed by the sample can be measured. The intensity of the absorption is related to the concentration of the component. The more energy that is absorbed, the more of that particular functional group exists in the sample. Results can therefore be numerically quantified. Further, the absence of an absorption band in a sample can often provide equally useful information.
Intensity and frequency of sample absorption are depicted in a two-dimensional plot called a spectrum. Intensity is generally reported in terms of absorbance, the amount of light absorbed by a sample, or percent transmittance, the amount of light that passes through it. In IR spectroscopy, frequency is usually reported in terms of wavenumbers, as defined above.
Infrared spectrometers may be built using a light source (e.g., the sun), a wavelength discriminating unit or optically dispersive element such as a prism, for example, and a detector sensitive to IR. By scanning the optically dispersive element, spectral information may be obtained at different wavelengths. However, one drawback to this approach is the moving parts associated with the required scanning operation. Such moving parts inherently limit the ruggedness and portability, for example, of such a device.
More recently, a Michelson interferometer has been used to generate a so-called interferogram in the IR spectrum, which later is subjected to Fourier transform processing such as a fast Fourier transform (FFT) to yield the final spectrum. In the IR range, such spectrometers are called FTIR interferometers, and the first commercially available appeared in the mid 1960xe2x80x2s. A representation of an FTIR interferometer is provided in FIG. 1.
The key components of FTIR interferometer 100 are IR source 110, interferometer (130, 140, 150), and IR detector 160. FTIR interferometer 100 provides a means for the spectrometer to measure all optical frequencies transmitted through sample 120 simultaneously, modulating the intensity of individual frequencies of radiation before detector 160 picks up the signal. Typically, moving mirror arrangement 150 is used to obtain a path length difference between two (initially) identical beams of light. After traveling a different distance than a reference beam, the second beam and the reference beam are recombined, and an interference pattern results. IR detector 160 is used to detect this interference pattern.
The detected interference pattern, or interferogram, is a plot of intensity versus mirror position. The interferogram is a summation of all the wavelengths emitted by the sample and, for all practical purposes, the interferogram cannot be interpreted in its original form. Using the mathematical process of Fourier Transform (FT), a computer or dedicated processor converts the interferogram into a spectrum that is characteristic of the light either absorbed or transmitted through sample 120.
The invention of FT spectroscopy has proven to be one of the most important advances in modern instrumentation development in the 20th Century. Optical spectroscopy utilizing the interference of light has made fast, sensitive detection of molecular vibration/rotation possible due to the large throughput and multiplex advantages provided by FT instrumentation. In Nuclear Magnetic Resonance (NMR) and mass spectroscopy where high-resolution spectra are required, FT instrumentation has also prevailed as the state of the art.
The same technological innovations that have made FT instruments those of choice for a generation of spectroscopists, however, have also made them extremely sensitive to their operating environment. For these reasons, FT interferometers are mostly limited to laboratory conditions which require the use of an optical bench to prevent vibration, and which also require stringent environmental controls to control temperature variations that adversely affect the interferogram by thermally inducing pathlength differences. While this type of scanning approach is workable, the signal-to-noise-ratios (SNR) obtainable often requires substantial signal averaging of multiple interferograms, thus making FTIR systems inherently slow, with reduced speed and lower reliability resulting from the numerous moving parts of these systems.
In spectroscopy, resolution is a measure of the ability to resolve or differentiate two peaks in the spectrum, where high resolution corresponds to a small wavenumber difference between the peak positions, and low resolution is associated with a larger wavenumber difference between the peak positions. Fourier Transform interferometers are capable of extremely high resolution, on the order of {fraction (1/1000)}th cmxe2x88x921, depending on the amount of possible movement of the mirror, or the pathlength difference that can be generated by the particular apparatus. xe2x80x9cLowxe2x80x9d resolution is generally considered to be in the range of 16-32 cmxe2x88x921, although no bright-line demarcation between xe2x80x9clowxe2x80x9d and xe2x80x9chighxe2x80x9d resolution exists, as resolution is chosen based on the required measurement and specific application. For typical chemical analysis and identification associated with FTIR, xe2x80x9chighxe2x80x9d resolution of 8 cmxe2x88x921 or better is common. Otherwise, chemical information is lost if the resolution is too low, as adjacent peaks identified with a particular chemical bond or vibration state may be xe2x80x9csmearedxe2x80x9d together and rendered indiscernible if a lower resolution is used.
The need for thermal stability, mechanical vibration isolation, and stringent optical alignment has put severe constraints on where and how FT instruments can be used and, in particular, has limited the portability of such instruments. If discussion is limited to FTIR interferometers, then an examination of the specific technology used in currently available instruments reveals where some of the shortcomings can be found. Table 1 compares the four most commonly used techniques for the operation of an optical interferometer, and their limitations.
FTIR has been applied to a variety of studies in industry, government, and academic laboratories, and has resulted in a major improvement upon conventional methods of performing analysis on a variety of samples. However, it has become clear that the moving mirror mechanism in a traditional interferometer has limited the design and construction of a more compact and portable FTIR. One potential solution attempted by Stelzle, Tuchtenhagen, and Rabolt (xe2x80x9cNovel All-fibre-optic Fourier-transform Spectrometer with Thermally Scanned Interferometerxe2x80x9d), was to construct an all-fiber-optic FT Spectrometer, which had no moving parts, and which was used to perform infrared spectroscopy.
In this feasibility study, an attempt was made to build an interferometer in the near-IR (10000-5000 cmxe2x88x921) range using fiber optics. Two carefully measured and cleaved optical fibers were used as the two light channels, or optical paths, with one fiber kept at ambient temperature while the other fiber was heated/cooled repeatedly. The resulting optical path difference (OPD) between the two fiber channels due to changes in both the length and the refractive index of the heated/cooled fiber causes interference in the combined channel. The heating/cooling cycle was used to generate an OPD of 3 cm, thus producing an interferogram with the power spectrum calculated accordingly.
However, the interference of two light beams in the optical fibers under different thermal and mechanical conditions turned out to be very complex. In contrast to the traditional Michelson interferometer, whose only source of optical path length difference comes from the geometric pathlength resulting from the moving mirror, a fiber-optic interferometer responds to any mechanical or thermal changes of the operating environment, which causes a scrambling or loss of the phase information necessary for interference to occur. It was concluded that although the fiber optics concept is a good one, a more prudent plan for a no-moving parts IR instrument had to be developed.
In surveying the literature, it became apparent that, without regard to the band of interest, e.g., visible, near-IR, or IR, other approaches to the construction of an FT interferometer with no-moving parts had also been attempted, as depicted in FIG. 2. Such approaches used either a linear array detector or a focal plane array (FPA) to collect interferograms. These designs involved the projection of the center portion of the interferogram onto the detector, and then used the xe2x80x9cimagedxe2x80x9d interferograms to calculate the power spectra after Fourier Transform processing. One difficulty of these conventional techniques is that the array detector size, its dynamic range, and the limited range of spectral response available limited the range of the interferograms that could be captured by the array detector.
In addition, even without moving parts, these approaches still rely upon calculation-intensive Fourier Transform processing to derive the power spectrum. Hence, there is still a need for a rugged, non-interferometric, no-moving part spectrometer in the mid-IR range.
Aside from Fourier Transform spectroscopy, spectroscopy based on dispersion also provides a possible implementation. In this approach, an optically dispersive element, such as a prism or diffraction grating, is used to separate the spectral frequencies present in the incident light radiation. The dispersive element was then rotated, in order to allow the various wavelengths present in the incident light to be detected.
IR spectroscopy based on dispersion became obsolete in most analytical applications in the late 1970xe2x80x2s due to its slow scan rate and lower sensitivity. It is well known that the scanning mechanism in a dispersive spectrometer, e.g., a moving prism, intrinsically limits both its ruggedness and optical throughput. The need for scanning comes from the fact that point detection of photons was the only available method at that time, and this was especially true in the IR range of the spectrum. Today, however, array detectors in the visible and near-IR range are widely available for area detection of photons. Charge-coupled-devices (CCD) capable of  greater than 80% quantum efficiency (QE) in the visible range have been made and utilized in many applications, such as the visible/near-IR camera aboard the Hubble Space telescope. As a result of this progress, CCD-based high performance spectrograph systems in the visible and near-infrared range can now be purchased through commercial suppliers. These systems provide alternatives to traditional FT interferometers.
However, the range of scientific problems which could now benefit from IR investigations has increased significantly, and applications involving samples which may change their position in the beam (e.g., vibrate or oscillate) while the spectrum is being recorded can not be routinely addressed using conventional FTIR instruments. The scanning architecture of FTIR instruments and the resulting modulation of the different optical frequency components can become modified further by a sample whose position fluctuates, and this can render the spectral information useless.
Hence, the need for a non-scanning instrument with convenient delivery and detection of IR radiation could never be stronger. For example, applications requiring on-line studies of micro mechanical deformation in polymer thin films during processing, in situ structural studies of aging in Light Emitting Diodes (LEDs), and the monitoring of inorganic (silicon, SiN, etc.) thin film growth on flexible polymer substrates would all benefit from an IR instrument with no moving parts, which as a consequence, will also be robust and portable. Such a portable instrument would facilitate materials research by providing a powerful new tool for thin film studies, especially those with fluctuating sampling geometries or in a remote sample location.
Further advantages for such a non-scanning, real-time instrument in the IR range could be found in environmental monitoring, including monitoring near military or civilian personnel during potential chemical or biological warfare attacks, due to the complex chemical compositions in such agents which show strong IR absorbance, and thus could be readily identified.
In spite of the inroads made in spectroscopy by spectrographs in the visible and near-infrared range due to the progress in CCD detectors mentioned previously, FT instrumentation still remains dominant in spectroscopy in the mid to far-infrared range and, therefore, instruments in this range are still extremely limited by the operating environment of the interferometer.
What is needed, then, is a robust, compact, and portable instrument in the IR range to address specific applications where sample fluctuations cause significant deterioration of the S/N in conventional FTIR spectra.
What is further needed is a portable and reliable IR spectroscope with no moving parts, and which is based upon IR focal plane array (FPA) technology.
Still what is further needed is a real-time, sensitive and relatively high-resolution apparatus and method for IR spectroscopic materials analysis, which does not rely upon interferometry or a calculation-intensive Fourier Transform approach, and which is relatively insensitive to harsh environments, including high vibration and wide temperature variations.
The present invention solves many of the aforementioned problems of providing a robust, high-resolution and sensitive apparatus and method for determining an IR spectrum of a sample material, without the use of moving parts, or calculation-intensive Fourier Transform interferometric techniques.
A first embodiment of the present invention includes an apparatus for determining an IR spectrum of a sample material based upon IR FPA technology to capture the IR spectral information, without utilizing a scanning mechanism, or any moving parts, and without the use of Fourier Transform signal processing.
An IR source is passed through a sample volume, where at least some of the IR energy is absorbed in the sample volume. The resulting IR signal is optically dispersed to spread the IR light into its respective wavelength components, and projected onto an IR detector having a plurality of detection elements. The detector output is further processed for display and analysis without interferometric techniques.
In a second embodiment, one or more optical fibers are used to couple the IR source through a sample volume, and into an optically dispersive element, and also into an IR detector. Such an embodiment may be used, for example, in remote-sensing applications, where the phenomena being evaluated are remotely located from the apparatus, particularly the IR detector. In an environmental application, which monitors smokestack emissions, for example, the sample volume to be analyzed may be hundreds of meters in the air. Fiber optical cabling may be used, as may telescopic optics to bring the experiment to the sensor.
In a first aspect of the first embodiment, an InSb focal plane array (FPA) is used to detect absorptions in the 3-5 xcexcm range and, in a second aspect of the first embodiment, a microbolometer-based FPA is utilized for the 7-13 xcexcm range. In yet another aspect of the first embodiment, an HgCdTd (MCT) array, or other InSb array having a wider or different spectral response may be used.
Signals from the samples can be collected by either of two methods. Signal collection by direct lens coupling may be used by coupling the signals into the spectrometer through an aperture. Alternatively, the coupling is also accomplished through the use of mid-IR optical fibers.
Use of optical fibers provide flexibility in placement of the apparatus, and allow remote sensing of, for example, smokestacks, and also allow easier implementation of multiple channel detection and chemical analysis.
The apparatus and method of the present invention does not require moving parts to determine spectral information. The method and apparatus are, consequently, well adapted to relatively harsh environments, such as, for example, high vibration environments in a manufacturing plant.
The method may also be used in various industrial applications to measure and detect the thickness, either in transmission or reflection mode, the chemical structure and orientation of coatings/films (solid, liquid, chemically bound, physically adsorbed) on liquid surfaces, including but not limited to water, oil and other solvents, and also to measure the thickness, orientation and chemical structure of films electrochemically deposited on solid substrates, including but not limited to metals and semiconductors.